1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, laser beam printer or the like, and more particularly to such an image forming apparatus employing an electrophotographic process.
2. Description of the Prior Art
Various apparatus utilizing an electrophotographic process, such as copier or laser beam printer are already in wide use. Among such apparatus there is already known an apparatus in which image information is optically given to a photosensitive member composed for example of CdS to form an electrostatic image thereon, which is then subjected to a development step such as jumping development to obtain a visible toner image for transfer onto a recording sheet.
In the development step of the above-mentioned process there is already known a method of applying an AC developing bias voltage across the photosensitive member and the developing unit to maintain appropriate density and tonal rendition. A sinusoidal voltage is usually employed as said AC bias voltage. Also there is known a method of super-posing a DC voltage to said AC developing bias voltage to maintain an appropriate level of deposition of toner onto the photosensitive member, or to prevent the toner deposition in the preparatory steps.
In such conventional developing unit, the use of sinusoidal developing bias voltage of a single frequency provides the advantage of easy control of wave form with respect to the oversheet or distortion and the possibility of employing a transformer and amplifier of a narrow band width for handling said AC bias voltage, but is associated with the following drawbacks.
The sinusoidal AC bias voltage gives rise to a large moving energy of the toner because of the large difference between the average level and the peak level of the signal and tends to cause discharge between the photosensitive member and the developing sleeve positioned opposed thereto because of the same reason. These phenomena become enhanced when a DC voltage is superposed as explained above, because of the higher peak value.